Mechanisms of anti-adult worm immunity and evasion in baboon schistosomiasis mansoni are being investigated. Baboons chronically infected with Schistosoma mansoni produce specific anti-worm antibodies and have lymphocytes that show depressed mitogenic and antigenic responses. The mechanism for depression is being sought as well as a role for anti-adult worm antibodies in reduced fecundity of female worms. Permissive and nonpermissive nonhuman primate hosts are being compared with respect to their antischistosome immune responses.